This invention relates to the processing of optical signals transmitted through optical fibers. More particularly the invention includes multiple optical fibers in a closely spaced side-by-side array on a substrate with an optical tap on each of the fibers. A second substrate with a tapped optical fiber is positioned in a face-to-face relationship with the first substrate. The second substrate is laterally adjusted so that the optical tap in the fiber on the second substrate may be selectively aligned with any one of the optical taps on the first substrate.
If the optical fibers on the first substrate are independent from one another the structure yields the optical equivalent of a single pole, multiple throw electrical switch which permits selective tapping of information into the fiber on the second substrate from any one of the fibers on the first substrate. Alternatively, the structure will selectively tap information from the fiber mounted on the second substrate into any one of the fibers mounted on the first substrate. If, in an alternative configuration, the first substrate mounts successive portions along the length of a single fiber, the apparatus forms a discretely variable fiber optic delay line.
Fiber optic switches, particularly for multi-mode fibers, are known in the art. Typically such switches are constructed by mounting multiple fibers on a first substrate such that cleaved or polished ends of the fibers are coplanar with one surface of the substrate. A second substrate mounts a fiber in a similar manner. The surfaces of the two substrates may be accurately transposed in order to align the butt end of the fiber in the second substrate with a selected one of the butt ends of the fibers in the first substrate, thus generating a switch. If such techniques were attempted with single mode optical fibers the degree of alignment required for low loss coupling would be extremely difficult to achieve. Thus, for example, if the core diameter of a single mode optical fiber is four microns, a misalignment of the two substrates by four microns would substantially eliminate coupling between the respective fibers. For this reason, a practical, less position-sensitive switch mechanism is needed, particularly for single mode optical fibers.
With regard to the use of this device as a delay line, it will be recognized that the advantages of fiber optic delay lines are well-known in the art. Thus, for example, tranversal filters capable of selectively filtering modulated light signals has been taught. Furthermore, the construction of transversal filters by helically wrapping a single fiber optic element around a series of v-grooves in a silicon chip, with taps at each groove, is known in the art. However, because no adjustment of the delay line lengths has been possible in prior art delay lines or transversal filters, the frequency vs. attentuation characteristics of prior transversal filters was, of necessity, determined at the time of construction of the filter. There is therefore a need for an adjustable fiber optic delay line so that, for example, the frequency response of a transversal filter utilizing the delay line may be adjusted.